Impacts of conservative endodontic cavities on root canal therapy compare to traditional endodontic cavities in premolars

Background: This study aims to compare the percentage of dentin removed, instrumentation efficacy, root canal filling and load at fracture between contracted endodontic cavities and traditional endodontic cavities in premolars. Methods: Forty extracted intact human first premolars were imaged with micro-CT and randomly assigned to CEC or TEC groups. Minimal CECs were prepared with the aid of 3D-printed guide template, canals were prepared with a 0.04 taper M-Two rotary instrument and cavities were restored with resin. The specimens were loaded to fracture in an Instron Universal Testing Machine after a fatigue phase. The data were analyzed by the independent samples T test and Mann-Whitney U test, appropriate post hoc tests. Results: In the premolars tested in vitro, CECs conserved coronal dentin in premolars with two dental roots but no impact on the instrument efficacy, the root canal filling or the biomechanical responses compared with TECs. Conclusion: The results of this study did not suggest that CEC could improve the fracture resistance of the endodontically treated premolars. The instrumentation efficacy and the percentage of filling material did not significantly differ between CECs and TECs in the premolars.

the concept of minimally invasive endodontics (MIE) are rapidly changing. MIE is characterized by "a systematic respect for the original tissue" and "preventing or treating disease with as little loss of original tissue as possible [6]. Contracted endodontic cavities (CECs), which were inspired by the concepts of MIE, emphasize endodontically treated tooth structure preservation, including pericervical dentin (PCD). The preservation of PCD is important for dental structure, and is associated with long-term survival benefit [7].
Because 3D technology could achieve precise design and positioning before operation, this technology has been applied in clinic and achieved good effects [12,13], the accuracy and safety for 3D printed template have been proved [14,15]. The objective is to design and make the 3D printed template for endodontic cavities, and explore the clinical significance of 3D printed template in endodontics.
Contracted endodontic cavities (CECs), as an alternative to traditional endodontic cavities, have been researched widely, so far, the outcomes and fracture resistance of CECs on root canal preparation still limited and controversial. In this study, systemic measurement including the percentage of dentin removed, instrumentation efficacy, the increased canal volume and surface areas, the increased sectional area, the percentage of the filling material and fracture resistance of premolars were provided.

Selection of teeth
The present study was approved by the Ethics Committee of the Hospital of Stomatology, Guangzhou Medical University (number KY2017012). Collected extracted human first premolars from orthodontic tooth extraction in oral and maxillofacial surgery, patients were requested if they could provide their extracted premolars to be used as specimens.
When they gave consent, they were asked to sign an agreement form to indicate that they were willing to provide their extracted premolars for specimens. Soft and hard tissue residuals on the surfaces of the teeth were removed using an ultrasonic scaler. All teeth had a fully formed apex without any defects or cracks on the surface and had no history of restoration. A curvature of 0-20°according to Schneider [16] on buccolingual and mesiodistal radiograghs was selected. Because the selected teeth were similar dimensions, there was no statistically significant difference (P>.05) in BL, MD or tooth root length between the CEC and TEC groups. The teeth were kept in 1% chloramine T trihydrate at room temperature until use.

Manufacture of 3D-printed splint guide
The guided access cavity was prepared using cone-beam computed tomography and optical surface scans. A high-resolution cone-beam computed tomography (CBCT) scan was taken to determine the exact location of the root canal, the drill was virtually superimposed on the root canal to plan the CEC outlines by projecting the access trajectory in each canal orifice that required the least tooth structure removal in Simplant (Simplant, Materialise Dental, Leuven, Belgium) (Fig. 1). The data were then imported into Freeform (Geomagic Freeform, 3D Systems, Morrisville, North Carolina, USA). According to the location of the drill in Simplant, we made a guide template with straight-line pathways into canals. The digitally designed template was exported as a STL-file, and then was sent to a 3D printer (3D System 3510HB, 3D Systems, Morrisville, North Carolina, USA).

TEC and CEC preparation
In CEC preparation, a 3D-printed template was positioned on the tooth model, and a guiding sleeve was placed on the hole. CECs were drilled with long diamond burs (MANI SF-11, MANI INC, Japan) at high speed, the CEC access attested the distal and mesial accesses could directed towards their respective orifices, which kept back the truss of dentin between the cavities (Fig. 2). In the TEC group, conventional access cavities were prepared. After initial preparation with pathfile instruments (Dentsply Maillefer, Ballaigues, Switzerland), canals were prepared with .04 taper M-Two rotary instruments (VDW company, Germany) to size 35#. These instruments were used in a standard technique, The canals were irrigated with 3 ml of 5.25% sodium hypochlorite between use of each instrument, and then, each canal was irrigated with 5% NaOCl followed by irrigation for 30 seconds with ultrasonic oscillation tip (K15/21-25, SATELEC, France) coupled with an ultrasound device (SATELEC P5XS, Merignac, France) at power 7. After cleaning and shaping, the teeth were imaged again with micro-CT imaging at 20 µm to capture the instrumented canal shape (posttreatment scan). All canals were obturated with gutta-percha cones (Dentsply Sirona) and AH plus sealer (Dentsply DeTrey, Konstanz, Germany). The thermoplastic continuous wave of condensation technique was used for obturation using a B&L-beta Gutta Percha Heating System (B&L Biotech, Inc, Korea). Smart Dentin Replacement (Dentsply, DE, USA) was used to imitate the lost dentin tissue and 2mm composite resin restorative material (Gradia Direct Posterior) was placed on the canal opening. The teeth were stored in physiological saline at 37°C for one week. Each specimen was subjected to micro-CT (SkyScan 1172; Bruker micro-CT, Kontich, Belgium) before and after instrumentation and irrigation as well as after obturation.

Load at fracture
After root canal filling and micro-CT scanning, teeth with a 3D printed cylindrical lampstand were mounted in an Instron Testing machine (E3000, Instron, High Wycombe, UK). The specimens were subjected to 500000 loading cycles in the Instron Testing machine (E3000) axial forces, directed at a 135 angle from the long axis of the tooth [17], between 5 N-50 N at 15 HZ to simulate approximately 2 years of chewing function [18,19].
After this fatigue phase, the specimens were placed in the Instron Universal Testing Machine (E3366, Instron, MA, America). Each tooth was loaded at the central fossa at 135°f rom the tooth long axis to simulate a maximum bending motion of the tooth at buccal cervical areas. 9 A continuous compressive force was applied with a 2-mm spherical crosshead at 1 mm/min until failure occurred, which was defined as a 25% drop in the applied force [20] (Fig. 3A). The load at fracture was recorded in Newton (N), and the type of fracture was recorded (Fig. 3B).

Evaluation methodology
Canal and crown boundaries were demarcated at the buccallingual level of the cementoenamel junction in single-rooted premolars, and canal boundaries were demarcated in root separation in premolars with two dental roots.

Statistical Analysis
The data were analyzed using IBM SPSS Statistics 16 software (Armonk, NY, USA), it were compared with independent samples T tests and the Mann-Whitney U test, P < .05 was considered significant.

Results
The percentage of dentin removed in the premolars with two dental roots in the CEC group (3.85% ± 0.42%) was significantly smaller (P < .05) than in the TEC group (4.94% ± 0.5%). The percentage of dentin removed has no significant differences (P > .05) between the TECs (3.4%±0.13%) and CECs (2.98% ± 0.12%) in the premolars with one dental root.
The UCW after instrumentation for TECs (16.43% ± 6.56%) was significantly lower (P< .05) than the UCW (24.42% ± 9.19%) for CECs in single-rooted premolars. The UCW in premolars with two canal roots did not differ significantly (P > .05) between the TEC (21.28% ± 8.91%) and CEC (18.62% ± 5.85%) groups (Table 1). No significant differences were observed in the increased canal volume and surface areas in premolars between the TEC and CEC groups (P > .05). In the premolars with two dental roots, the increased sectional area of 1 mm, 3 mm, and 5 mm from the major apical foramen was significantly greater (P < .05) in the CEC group than in the TEC group. The deviation of the central point after instrumentation for TECs was significantly smaller (P < .05) than that for CECs.
There was no significant difference (P > .05) between the TEC and CEC groups in the increased sectional area and the deviation of the central point in single-rooted premolars.
Micro-CT analysis revealed that there were no differences between the CEC groups and the TEC groups in the percentage of filling material and voids (P > .05) ( Table 2). In general, the mean load at failure of premolars did not significantly differ between the CEC and TEC groups and there was no significant difference in the type of fracture (P > .05) ( Table 3).

Discussion
Guided endodontics printed templates have been used to locate all root canals in the apical third of teeth with pulp canal calcification and apical pathology with the aid of 3D printing technology and digital dentistry [15,21]. In this study, we used guided endodontics printed templates for minimal cavity access, which acquired the least tooth structure removal and projected the access trajectory to each canal orifice.
A previous study reported that CEC seems to exhibit a better preservation of the original canal anatomy particularly at the crown level including incisors, premolars and molars with TECs [20]. The conservative endodontic cavity, which could keep back the truss of dentin between the cavities, could save more dental tissue in premolars with two dental roots. Although more tooth tissue was retained, there was no obvious increase in the fracture resistance.
The complete cleanness of root canal was still the primary objective for nonsurgical root canal therapy. On the basis of the experimental data, the instrumentation efficacy was more effective for TECs than for CECs in single-rooted premolars. Because contracted endodontic cavities are smaller than traditional endodontic cavities, the instrument could not shape the entire wall of the canal into a straight line. With the aid of the 3D printed template and the straight line of contracted endodontic cavities, the instrumentation efficacy of TECs and CECs showed no significant difference in premolars with two canal roots (p > .05). According to the report [22], the instrumentation efficacy did not differ significantly (P > .15) between CECs and TECs in any of the roots or canal levels in maxillary molars. Another research found there was no significant difference in remaining pulp tissue between the TEC and CEC groups within the MB or ML root canal at any of 1/3 of the root [23].
In the premolars with two dental roots, the increased sectional area was significantly bigger (P<.05) in the CEC group than in the TEC group. It is probably because when we design the canal orifice in the premolars with two dental roots, the drill was virtually superimposed on the root canal with inclination angle in coronal, because when we design a straight-line pathways into canals, the orifice would be in the buccal cusp, we shifted a little toward the middle to avoid the destroy of marginal ridges. Because of the inclination angle in coronal, the deviation of the central point after instrumentation for CECs was significantly bigger (P<.05) than that for TECs. Mario A et all also reported TECs showed more preservation of the original root canal anatomy with less apical transportation than CECs [24].
Micro-CT allowed the 3D anatomy assessment of root canal fillings and voids, and the results obtained in our study did not show obvious differences in the percentage of root canal filling in the CEC and TEC groups. Although the entrance of the pulp chamber is smaller in CECs, with the straight-line pathways into canals, the root canal filling can be completed for both CECs and TECs equally.
Numerous studies provided CEC preparation did not increase the fracture strength of teeth compared with TEC preparation [22,25,26], this result corroborate with those above researches. The researchers have found the endodontic procedures do not weaken teeth with intact marginal ridges [27], the CEC and TEC groups were both prepared with intact marginal ridges and there were no significant differences (P>.05) on biomechanical responses between the premolars in the CEC and TEC groups. At the same time, the result was opposite to Krishan's research [28],which reported CEC increased fracture resistance in premolars and mandibular molars, First of all, it is probably because that we simulated clinical treatment procedure to restore the access cavities with resin before fracture resistance test; secondly, the angle of the tooth loaded at the central fossa from the tooth long axis and the spherical crosshead was different in this study; thirdly, single-rooted premolars and premolars with two dental roots were differentiated; fourthly, each sample have been tested by fatigue cycle test. All of these factors have the potential affection on the final results; In addition, the load type was comparable to that experienced in the mouth and human teeth was subjected to forces in different directions at the same time; and teeth have irregular shapes; the experimental data acquired were just in one direction and the results were for reference.

Conclusion
Within the limitations of this study, the current results did not show obvious benefits associated with CEC group compared with TEC group. Although CECs could conserve more tooth hard tissue, the results of this study did not suggest that CEC could improve the fracture resistance of the endodontically treated premolars. The instrumentation efficacy and the percentage of filling material did not significantly differ between CECs and TECs in the premolars. Future experiments with bigger sample sizes and long-term clinical studies were encouraged to carry out on this topic.

Ethics approval and consent to participate
This study was approved by the Ethics Committee of the Hospital of Stomatology, Guangzhou Medical University and written informed consent was obtained from the participants.

Consent to publish
The authors agree to publication in the journal.

Availability of data and materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request

Competing interests
The authors declare that they have no competing interests.